People like to build airports on, or very close to, sites that have been used for millennia as resting and refueling stopovers by migrating birds. And this creates a lot of conflicts. The Bird Strike Committee USA estimates almost $1 billion in damages occur annually to American civil and military aircraft due to wildlife and bird strikes. These strikes have also caused the deaths of more than 250 people since 1988.
Yet, since scarecrows are something of a joke, is it even possible to reduce the chances that a bird strike will occur? Airport operations managers think so: they have adopted a variety of methods to convince birds to leave airports, ranging from pesticides and pyrotechnics to firearms and falconry, and even including recordings of bird distress calls. But the evidence indicates that these primitive methods don’t work for very long.
“You set out propane cannons, they’ll habituate. You broadcast predator calls, they’ll learn to ignore them. They’ll even get used to packs of angry wiener dogs”, said Mark Hinders, a professor of applied science at the College of William & Mary, and co-author of a paper that was recently published in the journal, Ecological Applications (ref).
“About the only things that work to exclude birds are nets, guns and poison”, said Professor Hinders in a statement. “But those are expensive and/or bad manners.”
Such exclusion methods also can be illegal.
This situation is made more complicated because finding suitable migratory resting and refueling sites is exhausting work, so many migratory bird species are extraordinarily faithful to their ancient stopovers (ref). This is particularly true for larger birds, such as ducks, geese and swans, which arrive every year on schedule, even after their stopovers have been paved over and crammed full of people, cars and aircraft.
This problem inspired Professor Hinders and John Swaddle, a professor of biology, also at the College of William & Mary, to collaborate on a project to develop an effective, non-lethal bird deterrent. To do this, the professors relied upon their knowledge of natural history traits shared amongst birds. Basically, and with rare exceptions, birds are highly aural and vocal beings. Songbirds, for example, sing to attract a potential mate and to claim a territory. Birds also are very social, often forming flocks as they go about their daily business. These flocks of birds chatter amongst themselves whilst feeding, and commonly call out to their companions with warnings about danger.
Birds are sensitive to some noises and to particular sound frequencies. For example, Professor Swaddle had previously documented that eastern bluebirds, Sialia sialis, are disturbed by noises with certain frequencies produced by traffic and machinery (ref). Aviary studies of common (European) starlings, Sturnus vulgaris, showed similar results.
In view of this information, Professor Hinders and Professor Swaddle decided to design an apparatus that short-circuits these avian conversations. This device, which they call a “Sonic Net”, consists of conventional and directional (parametric) speakers that produce a narrow beam of “colored noise”.
If you’re like me, you are probably wondering what is colored noise. Well, we’ve all heard of white noise — that’s the sound you hear when tuned in to an unused radio or TV channel. Basically, white noise is a steady stream of all, or almost all, auditory frequencies at the same energy level. In contrast to white noise, colored noise has more energy concentrated at either the high or low end of the sound spectrum, which changes the signal. For example, pink noise has stronger bass than does white noise, and it has both pitch and loudness fluctuations, so it rumbles like a train in the distance.
The end result is that colored noise is annoying noise. Colored noise interferes with normal bird chatter as they go about their business.
“It’s like the cocktail party problem”, said Professor Hinders. “You’re in a room and a lot of people are talking and it can be difficult to follow an individual conversation. It doesn’t even have to be especially loud. It’s just that all those other people’s words fill in the empty spaces. And so you go to a quiet room so that you can hear.”
But really, does pink noise deter birds?
To test the efficacy of pink noise as a bird deterrent, Professor Hinders and Professor Swaddle performed their studies on three sites at an active airfield near Newport News, Virginia. All sites were covered with a monoculture of short mown grass, were located close to a runway and to a service road, and all were approximately the same size (half a hectare each, which is roughly half the size of an American football field.)
The middle of the three sites was given long-term sound treatments, whereas the other two were used as reference sites:
At the beginning of the experiment, Professor Hinders, Professor Swaddle and their team installed a large outdoor speaker and amplifier onto the central study site and erected sound walls on three sides to prevent the sound from traveling onto the two reference sites.
“We surveyed bird activity at three places at the airfield four times a week for eight weeks”, said Professor Swaddle in a statement. During the entire eight week study period, the team conducted a 30 minute census of all birds present on, or flying over, each site. They counted both abundances and species richness. Two counts were conducted in the morning and two in the evening, and no counts were conducted during heavy rain. During the first four weeks of the experiment, the speakers simply stood on the test site, silent.
“The first four weeks gave us a baseline of bird activity”, said Professor Swaddle. Then, after four weeks had passed, the speaker was turned on and broadcast 2–10 kHz pink noise, uninterrupted, for 24 hours per day, from the end of week 4 to the end of week 8.
The response was dramatic.
At the two reference sites where no sounds were broadcast, the numbers of birds remained unchanged during the study period. However, as you can plainly see, the test site where the Sonic Net treatment was used showed an 82% drop in bird numbers within 50 metres of the speaker where the noise levels were over 80 decibels (this volume is similar to a passing freight train), and a 65% drop farther away, where the noise levels were between 65-80 decibels. Further, this effect remained as strong at the end of the study period as it was at the beginning (figure 2d), indicating that the birds were not habituating to the racket.
Why do birds leave Sonic Net areas?
The researchers think the noise makes the area near the Sonic Net speakers is too risky for birds to remain in for long because they cannot hear their companions’ alarm calls. Thus, the Sonic Net is a promising non-lethal technology for convincing birds to leave areas where their presence imperils them, or where they are not wanted.
Not only could this technology be used to keep birds and wildlife away from airports, but it could also be adapted to keep them off high-speed train tracks and possibly even away from highways. Additionally, this technology could keep birds and wildlife away from agricultural fields, orchards, salmon farms, garbage dumps, and golf courses, and could be used to warn birds and bats away from wind and solar farms, and from glass buildings.
“The new football stadium in Minneapolis happens to be right in the major flyway for lots and lots of birds and apparently they are building this with lots of glass and they have to do something to head off a plague of bird deaths. A technology like this might be just what the NFL needs to avoid yet another run of bad press”, said Professor Hinders.
It would appear that the applications for developing and using this technology are limited only by the user’s imagination.
“It could be pigeons pooping on the cars in your parking lot. It could be gulls pooping on your yacht”, said Professor Hinders helpfully.
Further, this technology could be refined to target specific species and areas. Already, the team is working on special speaker designs and is identifying precise sound frequencies that can target particular species, whilst allowing other species to remain undisturbed — a scenario that could greatly help, say, farmers who wish to encourage beneficial species (more here).
But of course, noise pollution is always a concern, so targeting specific areas is important.
“If you do it just right you can get that narrow beam of sound to cancel itself out after it propagates a certain distance”, said Professor Hinders. “This control over where the sound goes allows us to cover a particular region with a blanket of sound. Inside that area the birds can’t communicate, so they leave. Outside that area, nothing — so we don’t generate any noise pollution.”
Correction: An earlier version of this article indicated over 250 deaths per year have been caused by bird strikes since 1988. The correct figure is over 250 total since 1988.
For the complete Forbes/Science article including pictures and graphs go to: http://www.forbes.com/sites/grrlscientist/2016/04/18/acoustic-scarecrows-a-safe-humane-way-to-reduce-bird-strikes/#6716e3203fbf
Members can access the scientific article in the Wildlife Society Bulletin in the BSAC Library